There are several synonyms for the phrase "plunging back." The term can refer to a sudden, steep drop or descent, and synonyms for this usage include tumbling, plummeting, or free-falling. Alternatively, the phrase can denote a sudden return to a previous state or activity, such as going back to work after a vacation, returning to an old habit, or resuming a former relationship. Synonyms for this usage could include re-entering, returning, or resuming. However, the exact synonym may depend on the context in which the phrase is used.